Transitionals are usually regarded as
extinct fossil species that occupy a space between major groups and
which represent the process of transition between these groups. So, for
example, the transition between marine and land vertebrates is
represented wonderfully by a number of clear transitionals such as Acanthostega gunnari, Ichthyostega , Tulerpeton, Pederpes finneyae and Tiktaalik (see also Jennifer Clack’s superb book on the marine:land transition, Gaining Ground).
Transitionals are, of course, not just represented in fossils – all
species are in transition between the ancestral form from which they
came and the evolved form to which they are headed.

A recent paper in Science
(Nakabichi et al, Science 314, 267)
seems
to me to represent a different sort of transition in action. Lynn
Margulis (whose views on other matters such as the Gaia hypothesis and
her claim that there is no scientifically demonstrated link between HIV
and AIDS shows just how much tolerance good science shows to those with
a mixture of good and bizarre ideas) developed the now widely accepted
idea that organelles (such as mitochondria which have their own DNA) in
the cells of eukaryotes (animals, plants, fungi and protists)
originated as bacterial endosymbionts. Endosymbionts are separate
organisms that live in symbiosis with a host cell – ie they provide
some benefit that the host cell needs and in turn are supported and
protected by the host cell. There is strong evidence that mitochondria
were originally free-living bacteria, which invaded host cells, perhaps
originally as parasites. Subsequently a mutually beneficial
relationship developed (there are a vast number of mutually beneficial
relationships between bacteria and host organisms, with bacteria
providing benefits by their action from mammalian guts to the roots of
plants). Endosymbionts make their living on the same principle except
that they live within the cells of their host organism. Long lasting
endosymbiotic relationships become very close – they get to the point
where neither host nor bacterial invader can live without the other.
There is good evidence that the bacterial invader over time abandons
much of the basic physiology and genetic makeup that enabled it to live
independently, because the host cell provides many of those basic
functions. Indeed, there is evidence that lateral processes transfer
some of the genetic material of the endosymbiont from the genome of the
organelle to the nuclear genome of the cell. For example go here.

Endosymbionts
are well known today. For example, endosymbionts are known to exist in
many varieties of insect cells. In most cases the endosymbionts are
restricted to specialised cells called bacteriocytes. They reproduce
through generations of the host cells like organelles. The
endosymbionts have massively reduced genome sizes and a big bias of
nucleotide composition (the four nucleotides, A, T, G and C are
approximately equally represented in the genomes of free living
organisms but in endosymbionts and organelles the GC content is
significantly reduced). Examples of endosymbiotic bacteria in insects
include Buchnera, Blochmannia, Wigglesworthia and Baumannia. Nakabachi
et al have just published a fascinating short paper in
Science in which they report the sequencing of an endosymbiont, called
Carsonella ruddii, that is found in all species of a type of insect
called psyllids that feed on plant sap (Pachypsylla venusta). The
characteristics of this bacterial symbiont lie way beyond that of other
known insect endosymbionts.

How? Well first of all the genome of
Carsonella is tiny – it consists of 160 kilobases (which is a third of
the smallest previously known bacterial genome), and it contains only
182 genes most of which have some physical overlap with one another. It
has a very low GC - guanine/cytosine - content at only 16.5%, way below
that of other known organisms. Carsonella has lost all of its genes for
many categories that free-living bacteria need such as the creation of
a cell envelope and the genesis of nucleotides and lipids. Its genome
lacks many genes that are necessary for biological processes of
free-living bacteria. It seems that the host cell compensates for this
lack of apparently critical function. On the other hand Carsonella is
rich in genes to synthesise essential amino acids in which the food
(plant sap) of the host insect is poor – this is evidence of the
positive function of Carsonella to its insect host. Carsonella is so
reduced and so utterly dependent on its host nuclear genome that it can
be regarded as a transition between an obligate endosymbiont and a
eukaryotic organelle. It is a genuine transitional on its way from
bacterium to organelle. Never let creationists tell you that there are
no transitionals.